unittests.cfg: Make extra_params specification uniform

Currently, extra_params is using ":" rather than "="
on unittests.cfg. Let's fix this and make all configuration
assignments uniform on that file.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>